For the manufacturing of integrated circuit having small critical dimensions, for example, smaller than about 28 nm, resistors are typically formed using doped OD regions. Due to the low sheet resistance of the doped OD regions, resistors typically occupy large chip areas. In some circuits in which large resistors are needed, the chip areas occupied by the resistors become a significant portion of the overall chip areas. For example, in electro-static discharge (ESD) circuits or resistor-capacitor (RC) timing circuits, the lengths of the resistors may be as great as hundreds of microns. Accordingly, a resistor may include a plurality of doped strips that are separated from each other by isolation regions, such as shallow-trench isolation (STI) regions. The plurality of doped strips is connected in series. These resistors may occupy up to a third of the input/output circuit area. In addition, dummy poly silicon needs to be inserted into the resistor regions for the purpose of process uniformity. This causes the occupied chip area of the resistors to be increased even more.
There are other resistor structures. For example, polysilicon resistors may be formed as a plurality of polysilicon strips connected in series. The polysilicon strips are formed directly over STI regions in order to be electrically insulated from each other. These resistors, however, also occupy large chip areas.